1. Field of the Invention
The present invention relates to a water base ink for ink-jet recording to be used for an ink-jet recording apparatus.
2. Description of the Related Art
In the ink-jet recording system, ink droplets are formed, for example, by means of the electrostatic attraction system, the system in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element, and the system in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are adhered to a recording objective material such as paper to perform the recording.
In the ink-jet recording system as described above, the ink droplets are intermittently discharged at a high speed by repeating the drastic or abrupt compression of the ink in the minute structure. Therefore, the cavitation tends to occur. As a result, the dissolved gas, which has been dissolved in the ink, remains as bubbles in the ink flow passage in some cases. If the ink flow passage is contaminated with the bubbles, the following problem arises. That is, the transmission of the pressure is inhibited by the bubbles, and it is impossible to discharge the ink stably.
Those known as the water base ink for ink-jet recording to be used for the ink-jet recording system include those obtained by dissolving or dispersing water-soluble dyes or water-insoluble coloring agents in liquid media composed of water and water-soluble organic solvents. Recently, the high quality level is required for all of the qualities including, for example, the printing quality, the weather resistance, the long term stability, and the discharge performance. The attention is directed to the self-dispersing carbon black which is extremely superior in weather resistance and which is excellent in the printing quality and the long term stability.
However, the ink, which is based on the use of the self-dispersing type carbon black, is apt to cause the problem of ink discharge failure, because the minute bubbles, which are adhered to the surface of the porous carbon black, prompt the generation of the bubbles on account of the cavitation. Further, the carbon black particles cause a large flow resistance in the minute ink flow passage, and the pressure loss is increased when the ink is subjected to the purge operation. Therefore, a problem arises such that the normal state is not restored when the discharge failure arises.
The following method is widely utilized in order to solve the problem of the ink discharge failure as described above. That is, the ink flow passage is prevented from the adhesion of the bubbles by adding a surfactant or the like to improve the wettability of the interior of the ink flow passage so that the generated bubbles are discharged with ease. However, this method has involved the following problem. That is, the wettability on the recording paper is simultaneously enhanced, and the contour of the image area is blurred to cause the feathering. It is noted that the feathering resides in the following phenomenon. That is, when the printing is performed on the regular paper which is not surface-coated, then the ink is permeated along paper fibers existing in an irregular form, and an image, which is fuzzy and indistinct, is formed. In general, the feathering tends to occur when the wettability of the ink with respect to the paper surface is more increased. The feathering is the great hindrance when it is intended to perform the high quality printing. On the other hand, if the amount of addition of the surfactant is suppressed in order to avoid the feathering, the ink discharge failure consequently takes place, because the wettability of the ink with respect to the ink flow passage is insufficient.
The generation of the bubbles and the growth of the generated bubbles can be inhibited by decreasing the amount of the dissolved gas in the ink. Therefore, a method is widely utilized, in which a degassing step is added after the production of the ink. Japanese Patent No. 2696992 discloses an example in which the dissolved gas is removed by effecting the heating at 70° C. for 160 hours. However, in the case of this method, when a long period of time elapses after the degassing, the degassed state of the ink is extinguished due to the contact with the air. Therefore, a problem arises in relation to the long term stability of the ink.